Ferroelectric capacitor, process for production thereof and semiconductor device using the same

ABSTRACT

A ferroelectric capacitor includes a pair of electrodes, and at least one ferroelectric held between the pair of electrodes, in which the ferroelectric includes a first ferroelectric layer having a surface roughness (RMS) determined with an atomic force microscope of 10 nm or more; and a second ferroelectric layer being arranged adjacent to the first ferroelectric layer and having an RMS of 5 nm or less. A process produces such a ferroelectric capacitor by forming a first ferroelectric layer on or above one of a pair of electrodes at a temperature equal to or higher than a crystallization temperature at which the first ferroelectric layer takes on a ferroelectric crystalline structure, and forming a second ferroelectric layer on the first ferroelectric layer at a temperature lower than a crystallization temperature at which the second ferroelectric layer takes on a ferroelectric crystalline structure.

The application is a division of prior application Ser. No. 10/743,814,filed Dec. 24, 2003 now U.S. Pat. No. 6,855,974.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefits of the priorityfrom the prior Japanese Patent Application No. 2003-002577, filed inJan. 8, 2003, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ferroelectric capacitor which showsless fatigue and is suitable as a large-capacity nonvolatile memory, aprocess for efficiently producing the ferroelectric capacitor, and ahigh-performance semiconductor device having the ferroelectriccapacitor.

2. Description of the Related Art

Ferroelectrics have excellent ferroelectricity, piezoelectricity andpyroelectricity and are widely used in, for example, a variety ofmemories, actuators, and sensors. For example, such ferroelectrics havebeen applied to nonvolatile memories by utilizing the hysteresis of theferroelectrics. Certain ferroelectric capacitors comprising a lowerelectrode, a ferroelectric, and an upper electrode arranged in thisorder on a substrate are known as nonvolatile memories.

Pb-containing materials such as Pb(Zr,Ti)O₃ [PZT] having excellentferroelectricity have been suitably used as materials for theferroelectrics. These ferroelectrics may be formed, for example, by asol-gel method, sputtering or metalorganic chemical vapor deposition(MOCVD). Among them, the metalorganic chemical vapor deposition (MOCVD)has been often employed, since it can yield ferroelectric crystals whichexhibit high ferroelectricity even when finely divided and have a highdensity while it also provides a good step coverage. Such aferroelectric capacitor has been conventionally formed by forming alower electrode with the use of a noble metal such as Pt or Ir or anelectrically conductive oxide such as IrO_(x), wherein x is more than 0and is 2 or less; forming a ferroelectric film of lead zirconatetitanate (PZT) on the lower electrode by MOCVD; and forming an upperelectrode on the ferroelectric film.

However, when the ferroelectric capacitor having the thus-formedferroelectric is applied to a nonvolatile memory and the ferroelectricundergoes repetitive switching process (repetitive polarizationreversal), it induces “fatigue (polarization fatigue)” in which thepolarization of the ferroelectric decreases. To reduce the “fatigue”,i.e., to improve fatigue properties, Japanese Patent ApplicationLaid-Open (JP-A) Nos. 10-173141, 2001-144264, 2001-267518, and2002-100740 each propose the use of an oxide electrode in theferroelectric capacitor. Another attempt has been made to form aferroelectric capacitor having an IrO₂/PZT/Ir multilayer structure byMOCVD, but the resulting ferroelectric capacitor does not havesufficiently improved fatigue properties.

SUMMARY OF THE INVENTION

Under these circumstances, an object of the present invention is toprovide a ferroelectric capacitor which shows reduced fatigue and issuitable as a large-capacity nonvolatile memory, a process forefficiently producing the ferroelectric capacitor, and ahigh-performance semiconductor device having the ferroelectriccapacitor.

The present invention provides a ferroelectric capacitor including apair of electrodes, and at least one ferroelectric held between the pairof electrodes, wherein the at least one ferroelectric has a firstferroelectric layer having a surface roughness (root mean square)determined with an atomic force microscope of 10 nm or more; and asecond ferroelectric layer being arranged adjacent to the firstferroelectric layer and having a surface roughness (RMS) determined withan atomic force microscope of 5 nm or less.

The ferroelectric in the ferroelectric capacitor includes the firstferroelectric layer arranged on or above a lower electrode of the pairof electrodes and having a rough surface, and the second ferroelectriclayer arranged adjacent to the first ferroelectric layer and having asmooth surface. The ferroelectric capacitor can minimize defects at theinterface between the second ferroelectric layer and the other electrode(upper electrode) of the pair of electrodes to thereby prevent chargesfrom being trapped in such defects. Thus, the ferroelectric capacitor isresistant to “fatigue” in which the polarization decreases uponrepetitive switching process.

The present invention further provides a process for producing aferroelectric capacitor including a pair of electrodes and at least oneferroelectric held between the pair of electrodes, the process includingthe steps of forming a first ferroelectric layer on or above one of thepair of electrodes at a temperature equal to or higher than acrystallization temperature at which the first ferroelectric layer takeson a crystalline structure displaying ferroelectricity; and forming asecond ferroelectric layer adjacent to the first ferroelectric layer ata temperature lower than a crystallization temperature at which thesecond ferroelectric layer takes on a crystalline structure displayingferroelectricity. According to this process, the first ferroelectriclayer has a crystalline structure but the second ferroelectric layerdoes not have a crystalline structure but an amorphous structure beforethe formation of the other electrode (upper electrode). Thus, theferroelectric capacitor is effectively prevented from inducing defectsat the interface between the upper electrode and the secondferroelectric layer.

The present invention still further provides a semiconductor deviceincluding a substrate and a ferroelectric capacitor arranged on or abovethe substrate, in which the ferroelectric capacitor is the ferroelectriccapacitor of the present invention. The ferroelectric capacitor isresistant to defects at the interface between the second ferroelectriclayer and an upper electrode arranged thereon and can significantlyreduce “fatigue”. The resulting semiconductor device having theferroelectric capacitor has a large capacity, shows less variation inpolarization even upon repetitive switching process (repetitivepolarization reversal), can be rewritten at high speed in a large numberof cycles and consumes less power. The semiconductor device is thereforesuitable as, for example, a large-capacity nonvolatile memory inpersonal digital assistants, memory backup for game machines, displays,personal computers, printers, televisions, digital cameras, and otheroffice automation appliances.

Further objects, features and advantages of the present invention willbecome apparent from the following description of the preferredembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a ferroelectric capacitor as anexample of the present invention.

FIG. 2 is a schematic view of a fine structure of a surface ofPb(Zr,Ti)O₃ [PZT] film formed by MOCVD in Example 1, as determined withatomic force microscope (AFM).

FIG. 3 is a graph showing the relationship between the number ofswitching process and the reversed (switched) electric charge Qsw inferroelectric capacitors according to Example 1 and Comparative Example1.

FIGS. 4 through 10 are schematic process drawings showing an example ofa process for producing a semiconductor device having the ferroelectriccapacitor of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Ferroelectric Capacitor

The ferroelectric capacitor of the present invention comprises a pair ofelectrodes and at least one ferroelectric held between the pair ofelectrodes and may further comprise additional layers between the pairof electrodes.

The pair of electrodes are not specifically limited, may be selectedaccording to purposes and can be, for example, a lower electrode and anupper electrode in combination.

Lower Electrode

The lower electrode for use in the present invention is not specificallylimited, may be selected according to purposes, and examples of itsmaterials are (1) a noble metal such as Pt, Ir, and Au, (2) Ni with atleast one element selected from Sc, Ti, V, Cr, Mo, Fe, Co, Cu, Y, Zr,Nb, Mn, Ta, W, Ir, and Pt, (3) electrically conductive oxides such asIrO_(x), wherein x is more than 0 and is 2 or less, RuO₂, SrRuO₃, andLa_(2-x)Sr_(x)CuO₄, wherein x is more than 0 and is 1 or less. Amongthem, Ir is preferred for better inhibition of diffusion of Pb and O.

The lower electrode may have either a single-layer structure or amultilayer structure. When Ir is used, the lower electrode may comprisea single layer of Ir or a multilayer having a substrate or a layer of,for example, Ti or Si and an Ir layer arranged on or above the substrateor the layer. The lower electrode preferably has a multilayer structurefor better alignment of the ferroelectric such as PZT.

A suitable example of the multilayer structure is an Ir/Ti comprising aTi layer about 10 nm thick and an Ir layer about 150 nm thick.

The thickness of the Ir layer in the single-layer structure or in themultilayer structure is not specifically limited, may be set accordingto purposes and is preferably from 10 nm to 1000 nm and more preferablyfrom 50 nm to 500 nm.

The lower electrode can be formed according to any procedure notspecifically limited, but is preferably formed by sputtering.

Ferroelectric

The ferroelectric comprises at least a first ferroelectric layerarranged on or above the lower electrode and a second ferroelectriclayer arranged adjacent to the first ferroelectric layer.

First Ferroelectric Layer

The first ferroelectric layer has a surface roughness RMS determinedwith atomic force microscope (AFM) of 10 nm or more. The firstferroelectric layer which has been formed, for example, by chemicalvapor deposition (CVD) generally has a surface roughness RMS of 10 nm ormore.

The first ferroelectric layer preferably has a perovskite crystalstructure. The crystals constituting the first ferroelectric layerpreferably have a columnar structure for higher density and higherstrength.

The perovskite crystal structure is represented by the formula: ABX₃. Inthe perovskite crystal structure, a cation in the A-site and an anion inthe X-site have similar sizes, and another cation in the B-site having asmaller size than the cation in the A-site resides in a cubic systemunit cell constituted by the A-site and X-site. Most of compounds havingsuch a perovskite crystal structure have a structure slightly distortedfrom an ideal cubic crystal structure at room temperature. Owing to thedistortion, i.e., asymmetry in structure, the perovskite crystalstructure exhibits a variety of functions.

Materials for the ferroelectric constituting the first ferroelectriclayer are not specifically limited, may be selected according topurposes, and examples are Pb(Zr,Ti)O₃ [PZT], SrBi₂Ta₂O₉ [SBT], andBi₄Ti₃O₁₂ [BIT]. Each of these materials may be used alone or incombination. Among them, Pb(Zr,Ti)O₃ [PZT] is preferred for its largerresidual dielectric polarization (remanence).

It is preferred that the first ferroelectric layer comprises Pb(Zr,Ti)O₃[PZT] having a perovskite crystal structure and the second ferroelectriclayer comprises Pb(Zr,Ti)O₃ [PZT] having a perovskite crystal structureconverted from an amorphous structure.

The first ferroelectric layer is preferably formed at a temperatureequal to or higher than a crystallization temperature at which the firstferroelectric layer takes on a crystalline structure displayingferroelectricity. The crystallization temperature varies depending onthe material for the ferroelectric. For example, when the firstferroelectric is Pb(Zr,Ti)O₃ [PZT], it is preferably formed at atemperature of 500° C. or higher and more preferably from 500° C. to700° C.

The term “crystal structure displaying ferroelectricity” as used hereinmeans and includes, for example, the perovskite crystal structure.

The first ferroelectric layer can be formed according to any procedurenot specifically limited, such as chemical solution deposition (CSD),metalorganic chemical vapor deposition (MOCVD), pulse laser deposition(PLD), sol-gel method, and sputtering. Among such procedures, MOCVD ispreferred for better step coverage and for higher density and higherstrength of the crystals.

Suitable source gas and MOCVD conditions for the formation of the firstferroelectric layer may vary depending on, for example, the type of thefirst ferroelectric layer. When the first ferroelectric layer comprisesPb(Zr,Ti)O₃ [PZT], a Pb source gas, a Zr source gas, and a Ti source gasmay be used as the source gas.

The Pb source gas includes, for example, Pb(DPM)₂; the Zr source gasincludes, for example, Zr(dmhd)₄; the Ti source gas includes, forexample, Ti(O-iPr)₂(DPM)₂, wherein DPM represents2,2,6,6,-tetramethyl-3,5-heptanedionato, dmhd represents2,2-dimethyl-3,5-heptanedionato, and O-iPr represents isopropoxido.

Each of the flow rates of the Pb source gas, the Zr source gas, and theTi source gas is from about 0.01 ml/min. to about 1.0 ml/min., andpreferably from 0.1 ml/min. to 0.5 ml/min.

The oxygen partial pressure in the source gas is not specificallylimited, may be set according to purposes and is, for example, fromabout 1 Torr to about 10 Torr (about 133 Pa to 1333 Pa), and preferablyfrom 3 Torr to 7 Torr (from 399 Pa to 933 Pa).

The source gas can be prepared according to any procedure notspecifically limited and may be prepared by dissolving materials for thesource gas in a solvent such as tetrahydrofuran (THF) to yield asolution and vaporizing the solution.

The solution can be vaporized using a conventional vaporizer.

The vaporized source gas is mixed with, for example, oxygen gas to apredetermined oxygen partial pressure and is then sprayed to the lowerelectrode using a showerhead or another device. Thus, the firstferroelectric layer is formed on or above the lower electrode.

The conditions for the formation of the first ferroelectric layer arenot specifically limited and may be set according to purposes. Asuitable reaction temperature varies depending on the material of thefirst ferroelectric layer. When the first ferroelectric layer comprisesPb(Zr,Ti)O₃ [PZT], the reaction temperature is generally from about 580°C. to about 620° C.

The thickness of the first ferroelectric layer is not specificallylimited, may be set according to purposes and is preferably from about10 nm to about 1000 nm and more preferably from 50 nm to 500 nm.

Second Ferroelectric Layer

The second ferroelectric layer is arranged adjacent to the firstferroelectric layer and has a surface roughness (RMS) determined by AFMof 5 nm or less, preferably 3 nm or less. More preferably, the secondferroelectric layer has a substantially flat or smooth surface.

The second ferroelectric layer having a surface roughness RMS exceeding5 nm may partially have excessively thin portions.

The second ferroelectric layer is preferably so arranged as to filldepressions on the surface of the first ferroelectric layer having asurface roughness (RMS) determined by AFM of 10 nm or more to therebyconstitute a substantially flat surface.

The second ferroelectric layer preferably has a perovskite crystalstructure converted from an amorphous structure. The secondferroelectric layer preferably has a granular structure as its crystalstructure so as to fill depressions on the rough surface of the firstferroelectric layer.

The second ferroelectric layer may be formed at a temperature lower thana crystallization temperature at which the second ferroelectric layertakes on a crystalline structure displaying ferroelectricity. Thecrystallization temperature varies depending on the material of thesecond ferroelectric layer. When the second ferroelectric layercomprises Pb(Zr,Ti)O₃ [PZT], the second ferroelectric layer ispreferably formed at a temperature lower than 500° C.

The material for the ferroelectric constituting the second ferroelectriclayer is not specifically limited, as long as it can have an amorphousstructure under some film-forming conditions, and can be the samematerial as the first ferroelectric layer. For better film formation andhigher strength of the ferroelectric, a ferroelectric having the samecomposition as the first ferroelectric layer is preferably used as thematerial.

The second ferroelectric layer preferably comprises Pb(Zr,Ti)O₃ [PZT],except with its A-site in the perovskite crystal structure doped with,for example, La, Sr, Ca, or Ba.

It is also preferred that the second ferroelectric layer comprisesPb(Zr,Ti)O₃ [PZT], except with its B-site in the perovskite crystalstructure doped with, for example, Nb, Bi, Ta or W.

Preferred embodiments of the ferroelectric capacitors of the presentinvention are, for example, (1) a ferroelectric capacitor in which thefirst ferroelectric layer comprises Pb(Zr,Ti)O₃ [PZT] having aperovskite crystal structure, and the second ferroelectric layercomprises Pb(Zr,Ti)O₃ [PZT] having a perovskite crystal structure whichhas been converted from an amorphous structure and has an A-site dopedwith at least one selected from La, Sr and Ca; and (2) a ferroelectriccapacitor, in which the first ferroelectric layer comprises Pb(Zr,Ti)O₃[PZT] having a perovskite crystal structure, the second ferroelectriclayer comprises Pb(Zr,Ti)O₃ [PZT] having a perovskite crystal structurewhich has been converted from an amorphous structure and has a B-sitedoped with at least one selected from Nb and Bi.

By doping the second ferroelectric layer with a dopant, Pb defects atthe interface between the upper electrode and the second ferroelectriclayer can be effectively inhibited.

The second ferroelectric layer can be formed adjacent to the firstferroelectric layer according to any procedure not specifically limited.Examples of the procedure are chemical solution deposition (CSD),metalorganic chemical vapor deposition (MOCVD), pulse laser deposition(PLD), sol-gel method, and sputtering.

Each of these procedures can be used alone or in combination. Amongthem, sputtering is preferred for easier formation of an amorphous filmwith less impurities.

Sputtering conditions for the formation of the second ferroelectriclayer on the first ferroelectric layer are not specifically limited andcan be selected from among normal production conditions forferroelectric films by sputtering.

For example, the Pb(Zr,Ti)O₃ [PZT] having an amorphous structure can beformed at normal temperature at an input power of sputtering of 1.5 kWusing a sintered target having a composition of (Pb,La,Ca,Sr)(Zr,Ti)O₃[PLCSZT] in an atmosphere of Ar gas. The pressure in a vacuum chamber ispreferably from 1 Torr to 10 Torr (133 Pa to 1333 Pa) and morepreferably from 1 Torr to 3 Torr (133 Pa to 399 Pa).

The amorphous structure of the second ferroelectric layer is convertedinto a perovskite crystal structure in the following manner. The upperelectrode is formed on the second ferroelectric layer having theamorphous structure to yield a ferroelectric capacitor, and the entireferroelectric capacitor is then subjected to a thermal treatment toconvert the amorphous structure into the perovskite crystal structure.

The thermal treatment is not specifically limited, may be selectedaccording to purposes and is preferably one of reduced-pressure rapidthermal annealing (RTA) and normal-pressure rapid thermal annealing(RTA) both at a temperature higher than the film forming temperature ofthe first ferroelectric layer.

Upon the reduced-pressure rapid thermal annealing (RTA), theferroelectric capacitor is heated at a temperature preferably 40° C. to100° C. higher, more preferably 50° C. to 90° C. higher, andspecifically preferably 60° C. to 80° C. higher than the film formingtemperature of the first ferroelectric layer.

The pressure of the reduced-pressure rapid thermal annealing (RTA) is,for example, from 0.1 Torr to 10 Torr (from 13.3 Pa to 1333 Pa).

Upon the normal-pressure rapid thermal annealing (RTA), theferroelectric capacitor is heated at a temperature preferably 70° C. to160° C. higher, and more preferably 80° C. to 120° C. higher than thefilm forming temperature of the first ferroelectric layer.

Upper Electrode

The upper electrode is not specifically limited, may be selectedaccording to purposes and can be, for example, one formed on or abovethe second ferroelectric layer by sputtering. Conditions for theformation of the upper electrode are not specifically limited and may beset according to purposes.

Materials for the upper electrode are not specifically limited, may beselected according to purposes, but suitable examples are IrO₂, RuO₂,SrRuO₃, La_(2-x)Sr_(x)CuO₄, wherein x is more than 0 and is 1 or less,and other oxides. Among them, IrO₂ is preferred for efficientlyinhibiting the diffusion of Pb.

The thickness of the upper electrode is not specifically limited, may beset according to purposes and is, for example, from about 10 nm to about1000 nm, and preferably from 50 nm to 500 nm.

The ferroelectric capacitor can have any structure not specificallylimited, such as a planar structure, a two-dimensionally stackedstructure, and a three-dimensionally stacked structure.

The ferroelectric capacitor can be produced by any process notspecifically limited and is preferably produced by the process forproducing a ferroelectric capacitor of the present invention mentionedlater.

The ferroelectric capacitors of the present invention can be suitablyused in various applications such as semiconductor devices requiringlarge-capacity ferroelectric capacitors and is specifically suitablyused in the semiconductor device of the present invention, mentionedlater.

Production Process for Ferroelectric Capacitor

In the process for producing a ferroelectric capacitor of the presentinvention, the first ferroelectric layer is formed on or above one(lower electrode) of the pair of electrodes at a temperature equal to orhigher than a crystallization temperature at which the firstferroelectric layer takes on a crystalline structure displayingferroelectricity; and the second ferroelectric layer is then formedadjacent to the first ferroelectric layer at a temperature lower than acrystallization temperature at which the second ferroelectric layertakes on a crystalline structure displaying ferroelectricity.

In the process, it is preferred that the first ferroelectric layer isformed on or above the lower electrode at a temperature of 500° C. orhigher and more preferably 500° C. to 700° C., and the secondferroelectric layer is then formed adjacent to the first ferroelectriclayer at a temperature lower than 500° C.

It is preferred that the upper electrode is formed on the secondferroelectric layer to yield a ferroelectric capacitor, and the entireferroelectric capacitor is subjected to a thermal treatment to therebyconvert the second ferroelectric layer from an amorphous structure to aperovskite crystal structure. The ferroelectric capacitor can bethermally treated under any conditions not specifically limited, asdescribed above.

Thus, the second ferroelectric layer does not have a crystallizedstructure at the time when the upper electrode is formed thereon, anddefects at the interface between the upper electrode and the secondferroelectric layer can be effectively inhibited.

The upper electrode alone may be etched before the thermal treatment ofthe entire ferroelectric capacitor. In this case, the area of theferroelectric capacitor to be heated is reduced, and the ferroelectriccapacitor can be thermally treated more efficiently, since theperipheral length of the ferroelectric capacitor becomes relativelylarger with respect to the area of the ferroelectric capacitor to beheated.

The first and second ferroelectric layers can be formed by any procedurenot specifically limited, and preferred examples of such procedures arechemical solution deposition (CSD), metalorganic chemical vapordeposition (MOCVD), pulse laser deposition (PLD), sol-gel method, andsputtering.

The first and second ferroelectric layers are preferably formed bymetalorganic chemical vapor deposition (MODVD), respectively, in whichthe forming temperature of the first ferroelectric layer is higher thanthe forming temperature of the second ferroelectric layer.

Alternatively, it is preferred that the first ferroelectric layer isformed on or above the lower electrode by metalorganic chemical vapordeposition (MODVD), and the second ferroelectric layer is formedadjacent to the first ferroelectric layer by sputtering.

An embodiment of the process for producing a ferroelectric capacitor ofthe present invention will be illustrated below.

For example, with reference to FIG. 1, a lower electrode 1 is formed bysputtering to a thickness of about 150 nm on a SiO₂ film 30 on a siliconsubstrate 100. A Pb(Zr,Ti)O₃ [PZT] film 2 a is formed by MOCVD on thelower electrode 1. More specifically, the Pb(Zr,Ti)O₃ [PZT] film 2 a isformed at a temperature of 620° C. feeding Pb(DPM)₂ as a Pb source at0.37 ml/min., Zr(dmhd)₄ as a Zr source at 0.31 ml/min., andTi(O-iPr)₂(DPM)₂ as a Ti source at 0.21 ml/min. at an oxygen partialpressure of 5 Torr (666 Pa). Each of these materials is dissolved intetrahydrofuran (THF) in a concentration of 3% by mole to yield asolution, and the solution is conveyed to a vaporizer. The solutioncontaining THF and the material is vaporized at 260° C. in thevaporizer, is mixed with oxygen gas to form a source gas, and the sourcegas is sprayed to the lower electrode using a showerhead. Thefilm-forming time of the Pb(Zr,Ti)O₃ [PZT] film 2 a is 480 seconds.

Next, a Pb(Zr,Ti)O₃ [PZT] film 2 b having an amorphous structure isformed at normal temperature on the Pb(Zr,Ti)O₃ [PZT] film 2 a. Morespecifically, the Pb(Zr,Ti)O₃ [PZT] film 2 b is formed by sputtering atan input power of 1.5 kW using (Pb,La,Ca,Sr)(Zr,Ti)O₃ as a target in anatmosphere of Ar gas. A chamber is evacuated and is adjusted to apressure of 0.5 Pa while supplying the Ar gas.

A film of IrO₂ as an upper electrode 3 is formed by sputtering to athickness of about 200 nm on the Pb(Zr,Ti)O₃ [PZT] film 2 b having anamorphous structure to form a ferroelectric capacitor 50.

The entire ferroelectric capacitor 50 is then subjected to rapid thermalannealing (RTA) at 725° C. to thereby convert the Pb(Zr,Ti)O₃ [PZT] film2 b from the amorphous structure to a perovskite crystal structure.Thus, the ferroelectric capacitor of the present invention can beproduced.

The process of the present invention can efficiently producehigh-performance ferroelectric capacitors in quantity.

Semiconductor Device

The semiconductor device of the present invention and its productionprocess will be illustrated below.

The semiconductor device of the present invention is not specificallylimited and may be selected according to purposes, as long as itcomprises a substrate and a ferroelectric capacitor arranged on or abovethe substrate, in which the ferroelectric capacitor is the ferroelectriccapacitor of the present invention.

The semiconductor device can be produced, for example, in the followingmanner.

With reference to FIG. 4, an element-separation (isolation) dielectricfilm is formed by local oxidation of silicon (LOCOS) on a surface of asilicon (Si) substrate 100 to define and separate an element region. Theelement-separation dielectric film can be formed by LOCOS or by forminggrooves on the silicon substrate and embedding a dielectric film in thegrooves. The silicon substrate 100 may be of either n-type or p-type.

A transistor having a side-wall dielectric film with a gate electrode 18and a source-drain doped layer is formed in the element region. Aninterlayer dielectric film 22 composed of a silicone oxide layer isentirely formed by CVD, followed by smoothing the surface of theinterlayer dielectric film 22 by chemical-mechanical polishing (CMP).

Next, contact holes reaching the source-drain doped layer are formed inthe interlayer dielectric film 22 by photolithography, and a Ti film anda TiN film are sequentially formed by sputtering on the entire surfaceof the resulting article to form a coherent layer comprising the Ti filmand TiN film. Next, a tungsten (W) layer is formed by CVD on the entiresurface of the coherent layer. Thus, the coherent layer and the tungstenlayer are formed on the interlayer dielectric film 22 and inside thecontact holes.

The coherent layer and the tungsten layer are polished bychemical-mechanical polishing so as to expose the surface of theinterlayer dielectric film 22 to thereby form electrically conductiveplugs 24 comprising the coherent layer and the tungsten layer embeddedin the contact holes, as shown in FIG. 4.

With reference to FIG. 5, a lower electrode 1 composed of Ir is formedby sputtering. A Pb(Zr,Ti)O₃ [PZT] layer 2 a is formed by MOCVD on thelower electrode 1 heated at 400° C. to 700° C.; a Pb(Zr,Ti)O₃ [PZT]layer 2 b having an amorphous structure is formed by sputtering on thePb(Zr,Ti)O₃ [PZT] layer 2 a; and an upper electrode 3 is formed on thePb(Zr,Ti)O₃ [PZT] layer 2 b having an amorphous structure. The entireferroelectric capacitor is subjected to rapid thermal annealing (RTA) at725° C. to convert the Pb(Zr,Ti)O₃ [PZT] layer 2 b from the amorphousstructure to a perovskite crystal structure to thereby crystallize thePb(Zr,Ti)O₃ [PZT] layer 2 b.

With reference to FIG. 6, the ferroelectric capacitor is etched andthereby yields a two-dimensionally stacked ferroelectric capacitor.

With reference to FIG. 7, a protecting film 4 composed of, for example,Pb(Zr,Ti)O₃ [PZT] is formed on the surface of the ferroelectriccapacitor. With reference to FIG. 8, an interlayer dielectric film 5composed of, for example, tetraethyl orthosilicate (TEOS) is formed onthe protecting film 4 and is then flattened by chemical-mechanicalpolishing (CMP). With reference to FIG. 9, a plug-contact area is openedin the interlayer dielectric film 5, a TiN/Ti layer 6 and a W layer 7are sequentially formed and are subjected to chemical-mechanicalpolishing (CMP) to form a plug. With reference to FIG. 10, a TiN/Tilayer 8, an Al layer (or Al—Cu layer) 9 and a Ti/TiN layer 10 aresequentially formed and are patterned and etched to form a wiring layer.Then, a cycle of interlayer formation, chemical-mechanical polishing(CMP), plug opening, plug formation, wiring formation, wiringpatterning, and wiring etching is sequentially repeated to form amultilayer structure.

Thus, the semiconductor device of the present invention having theferroelectric capacitor of the present invention is produced.

The semiconductor devices of the present invention have a largecapacity, show less variation in polarization even upon repetitiveswitching process, can be rewritten at high speed in a large number ofcycles and consume less power. Thus, the semiconductor devices aresuitably used in various fields and especially suitably used as, forexample, large-capacity nonvolatile memories in personal digitalassistants, memory backup for game machines, displays, personalcomputers, printers, televisions, digital cameras, and other officeautomation appliances.

The present invention will be illustrated in further detail withreference to several examples and a comparative example below, which arenever intended to limit the scope of the present invention.

EXAMPLE 1

A ferroelectric capacitor 50 shown in FIG. 1 was produced in thefollowing manner.

Initially, a lower electrode 1 was formed by sputtering to a thicknessof about 150 nm on a SiO₂ film 30 on a silicon substrate 100.

A Pb(Zr,Ti)O₃ [PZT] film 2 a was formed by MOCVD on the lower electrode1. More specifically, the Pb(Zr,Ti)O₃ [PZT] film was formed at atemperature of 620° C. feeding Pb(DPM)₂ as a Pb source at 0.37 ml/min.,Zr(dmhd)₄ as a Zr source at 0.31 ml/min., and Ti(O-iPr)₂(DPM)₂ as a Tisource at 0.21 ml/min. at an oxygen partial pressure of 5 Torr (666 Pa).Each of these materials was dissolved in tetrahydrofuran (THF) in aconcentration of 3% by mole to yield a solution, and the solution wasconveyed to a vaporizer. The solution containing THF and the materialwas vaporized at 260° C. in the vaporizer, was mixed with oxygen gas toform a source gas, and the source gas was sprayed to the lower electrodeusing a showerhead. The film-forming time of the Pb(Zr,Ti)O₃ [PZT] film2 a was 480 seconds.

The Pb(Zr,Ti)O₃ [PZT] film 2 a formed by MOCVD was observed on itssurface with an atomic force microscope(AFM) and was found to have asurface as shown in FIG. 2 with a surface roughness RMS of 13 nm.

Next, a Pb(Zr,Ti)O₃ [PZT] film 2 b having an amorphous structure andcontaining 3% by mole of La corresponding to an A-site of a perovskitecrystal structure was formed at normal temperature on theMOCVD-Pb(Zr,Ti)O₃ [PZT] film 2 a. More specifically, the Pb(Zr,Ti)O₃[PZT] film 2 b was formed by sputtering at an input power of 1.5 kWusing (Pb,La,Ca,Sr)(Zr,Ti)O₃ as a target in an atmosphere of Ar gas. Achamber was evacuated and was adjusted to a pressure of 0.5 Pa whilesupplying the Ar gas.

The Pb(Zr,Ti)O₃ [PZT] film 2 b having an amorphous structure wasobserved with an atomic force microscope (AFM) and was found to have asurface roughness RMS of 3 nm.

A film of IrO₂ as an upper electrode 3 was formed by sputtering to athickness of about 200 nm on the Pb(Zr,Ti)O₃ [PZT] film 2 b having anamorphous structure to form a ferroelectric capacitor 50. The entireferroelectric capacitor 50 was then subjected to rapid thermal annealing(RTA) at 725° C. to thereby convert the Pb(Zr,Ti)O₃ [PZT] film 2 b fromthe amorphous structure to a perovskite crystal structure. Thus, theferroelectric capacitor of Example 1 was produced.

COMPARATIVE EXAMPLE 1

A ferroelectric capacitor of Comparative Example 1 was produced by theprocedure of Example 1, except that a Pb(Zr,Ti)O₃ [PZT] film having anamorphous structure was not formed on the MOCVD-Pb(Zr,Ti)O₃ [PZT] film.

The fatigue of the ferroelectric capacitors of Example 1 and ComparativeExample 1 was determined by the following method. The results are shownin FIG. 3.

Fatigue

The polarization of a sample ferroelectric capacitor was reversed byapplying pulses at 3 V, and the switched charge Qsw was determined at1.8 V.

FIG. 3 shows that the ferroelectric capacitor of Comparative Example 1having no Pb(Zr,Ti)O₃ [PZT] film on the MOCVD-Pb(Zr,Ti)O₃ [PZT] filmshows a decreased Qsw, 40% of an initial Qsw, after 2×10⁸ cycles ofpolarization reversal; in contrast, the ferroelectric capacitor ofExample 1 having the Pb(Zr,Ti)O₃ [PZT] film with a perovskite crystalstructure converted from an amorphous structure on the MOCVD-Pb(Zr,Ti)O₃[PZT] film shows less decreased Qsw, i.e., 80% of the initial Qsw, evenafter 2×10⁸ cycles of polarization reversal, indicating that theferroelectric capacitor of the present invention exhibits reducedfatigue as compared with the ferroelectric capacitor of ComparativeExample 1.

EXAMPLE 2

Production of Semiconductor Device

The semiconductor device was produced in the following manner.

Initially, as shown in FIG. 4, an element-separation dielectric film wasformed by local oxidation of silicon (LOCOS) on a surface of a siliconSi substrate 100 to define and separate an element region. Theelement-separation dielectric film can be formed by LOCOS or by forminggrooves on the silicon substrate and embedding a dielectric film in thegrooves. The silicon substrate 100 may be of either n-type or p-type.

A transistor having a side-wall dielectric film with a gate electrode 18and a source-drain doped layer was formed in the element region. Aninterlayer dielectric film 22 composed of a silicone oxide was entirelyformed by CVD, followed by smoothing the surface of the interlayerdielectric film 22 by chemical-mechanical polishing (CMP).

Next, contact holes reaching the source-drain doped layer were formed inthe interlayer dielectric film 22 by photolithography, and a Ti film anda TiN film were sequentially formed by sputtering on the entire surfaceof the resulting article to form a coherent layer comprising the Ti filmand TiN film. Next, a tungsten (W) layer was formed by CVD on the entiresurface of the coherent layer. Thus, the coherent layer and the tungstenlayer were formed on the interlayer dielectric film 22 and inside thecontact holes.

The coherent layer and the tungsten layer were polished bychemical-mechanical polishing so as to expose the surface of theinterlayer dielectric film 22 to thereby form electrically conductiveplugs 24 comprising the coherent layer and the tungsten layer embeddedin the contact holes, as shown in FIG. 4.

With reference to FIG. 5, a lower electrode 1 composed of Ir was formedwith sputtering. A Pb(Zr,Ti)O₃ [PZT] layer 2 a was formed by MOCVD onthe lower electrode 1 heated at 400° C. to 700° C.; a Pb(Zr,Ti)O₃ [PZT]layer 2 b having an amorphous structure was formed by sputtering on thePb(Zr,Ti)O₃ [PZT] layer 2 a; and an upper electrode 3 was formed on thePb(Zr,Ti)O₃ [PZT] layer 2 b having an amorphous structure. The entireferroelectric capacitor was subjected to rapid thermal annealing (RTA)at 725° C. to convert the Pb(Zr,Ti)O₃ [PZT] layer 2 b from the amorphousstructure to a perovskite crystal structure to thereby crystallize thePb(Zr,Ti)O₃ [PZT] layer 2 b.

With reference to FIG. 6, the ferroelectric capacitor was etched andthereby yielded a two-dimensionally stacked ferroelectric capacitor.

With reference to FIG. 7, a protecting film 4 composed of, for example,Pb(Zr,Ti)O₃ [PZT] was formed on the surface of the ferroelectriccapacitor. With reference to FIG. 8, an interlayer dielectric film 5composed of, for example, tetraethyl orthosilicate (TEOS) was formed onthe protecting film 4 and was then flattened by chemical-mechanicalpolishing (CMP). With reference to FIG. 9, a plug contact area wasopened in the interlayer dielectric film 5, a TiN/Ti layer 6 and a Wlayer 7 were sequentially formed and were subjected tochemical-mechanical polishing (CMP) to form a plug. With reference toFIG. 10, a TiN/Ti layer 8, an Al layer (or Al—Cu layer) 9 and a Ti/TiNlayer 10 were sequentially formed, patterned and etched to form a wiringlayer. Then, a cycle of interlayer formation, chemical-mechanicalpolishing (CMP), plug opening, plug formation, wiring formation, wiringpatterning, and wiring etching was sequentially repeated to form amultilayer structure.

Thus, the semiconductor device of the present invention having theferroelectric capacitor was produced.

The present invention can solve the problems in conventionalferroelectric capacitors and can provide a ferroelectric capacitor whichexhibits reduced fatigue and is suitable as a large-capacity nonvolatilememory, a process for efficiently producing the ferroelectric capacitor,and a high-performance semiconductor device having the ferroelectriccapacitor.

While the present invention has been described with reference to whatare presently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

1. A process for producing a ferroelectric capacitor comprising a pairof electrodes and at least one ferroelectric held between the pair ofelectrodes, the process comprising the steps of: forming a firstferroelectric layer with a surface roughness of 10 nm (RMS) or more,directly on one of the pair of electrodes at a temperature equal to orhigher than a crystallization temperature at which the firstferroelectric layer takes on a crystalline structure displayingferroelectricity; and forming a second ferroelectric layer adjacent tothe first ferroelectric layer at a temperature lower than acrystallization temperature at which the second ferroelectric layertakes on a crystalline structure displaying ferroelectricity.
 2. Aprocess according to claim 1, wherein the first ferroelectric layer isformed at a temperature equal to or higher than 500° C., and wherein thesecond ferroelectric layer is formed at a temperature lower than 500° C.3. A process according to claim 1, further comprising: forming the otherone of the pair of electrodes on or above the formed secondferroelectric layer; and subjecting the entire ferroelectric capacitorto a thermal treatment to thereby convert the second ferroelectric layerfrom an amorphous structure to a crystalline structure.
 4. A processaccording to claim 1, wherein the first and second ferroelectric layersare formed by at least one procedure selected from the group consistingof chemical solution deposition (CSD), metalorganic chemical vapordeposition (MOCVD), pulse laser deposition (PLD), sol-gel method, andsputtering.
 5. A process according to claim 1, wherein the first andsecond ferroelectric layers are formed by metalorganic chemical vapordeposition (MOCVD), and wherein the first ferroelectric layer is formedat a temperature higher than a temperature at which the secondferroelectric layer is formed.
 6. A process according to claim 1,wherein the first ferroelectric layer is formed by metalorganic chemicalvapor deposition (MOCVD), and wherein the second ferroelectric layer isformed by sputtering.
 7. A process according to claim 1, wherein saidferroelectric capacitor comprises at least one ferroelectric selectedfrom the group consisting of Pb(Zr,Ti)O₃ [PZT], SrBi₂Ta₂O₉ [SBT], andBi₄Ti₃O₁₂ [BIT].
 8. A process according to claim 1, wherein said firstferroelectric layer contains Pb(Zr,Ti)O₃ [PZT] having a perovskitecrystal structure and said second ferroelectric layer containsPb(Zr,Ti)O₃ [PZT] having a perovskite crystal structure converted froman amorphous structure.
 9. A process according to claim 1, wherein thesecond ferroelectric layer has a perovskite crystal structure, exceptwith its A-site doped with at least one selected from the groupconsisting of La, Sr, Ba and Ca.
 10. A process according to claim 1,wherein the second ferroelectric layer has a perovskite crystalstructure, except with its B-site doped with at least one selected fromthe group consisting of Nb, Ta, W and Bi.